Just updated to the Winter Update and the build menu disappears after a few seconds. Pressing 'B' doesn't bring it back. Mashing various buttons including 'Esc' made the 'B' button bring back the build menu, but it disappeared again after a few seconds. Will try restarting the game and see what happens. I just want to delete the random basket on the floor... lol
Silly me, I fixed it by disabling "Automatically Hide HUD (Experimental)".
Bug persists after restarting the game and travelling to another realm. Will try verifying steam files.